1. Field of the Invention
This invention relates to an apparatus for making a non-coating type magnetic recording medium by depositing a thin film such as a magnetic film onto a non-magnetic substrate in a vacuum.
2. Description of the Prior Art
Many of the conventional magnetic recording media are of the so-called coating type and made by using powdered magnetic materials such as magnetic oxide particles and magnetic alloy particles, for example, .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped Fe.sub.3 O.sub.4, Berthollide compounds of .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, Co-doped Berthollide compounds, CrO.sub.2 or the like. These powdered magnetic materials are dispersed in organic binders such as vinyl chloride-vinyl acetate copolymers, styrene-butadiene copolymers, epoxy resins and polyurethane resins. The dispersions thus obtained are then applied in coats on non-magnetic substrates and dried to form the magnetic recording media.
Recently, so-called non-coating type magnetic recording media using no binders have attracted attention because of their sbility to meet strong demand for high density recording. The magnetic recording media of this type have magnetic recording layers which consist on thin ferromagnetic metal films formed by a process such as vacuum deposition, sputtering or ion plating. Thus various efforts are being made to develop non-coating type magnetic recording media suitable for practical use.
To make a non-coating type magnetic recording medium, it has been proposed to vaporize a ferromagnetic metal and cause the resulting vapor stream of the ferromagnetic metal to impinge upon a substrate at an oblique angle, thereby forming a thin film of the ferromagnetic metal on the substrate. This oblique incidence vacuum deposition process is easy to conduct and can form a thin film exhibiting satisfactory magnetic characteristics by use of a relatively small apparatus.
In the conventional oblique incidence vacuum deposition process, a substrate is generally moved along the curved surface of a cylindrical cooling can positioned above the material to be evaporated and deposited. The vapor stream of the deposition metal material is caused to impinge upon the substrate at an extremely limited angle of incidence with respect to the substrate surface, thereby forming a thin ferromagnetic metal film on the substrate to a predetermined thickness. However, because the metal vapor stream is at an oblique angle with respect to the surface of the substrate, the thickness of the deposited thin film equals the product of the cosine of the angle of incidence and the thickness obtained when the angle of incidence of the vapor stream is zero, namely when the vapor stream impinges normal to the surface of the substrate. Accordingly, the deposition efficiency considerably drops as the angle of incidence of the vapor stream increases. Further, because of the geometrical arrangement of the substrate and the deposition material, the distance therebetween increases as the angle of incidence of the vapor stream increases, resulting in a still lower deposition efficiency. Furthermore, because the magnetic characteristics of the deposited film depend upon the angle of incidence (refer to Schuede: J.A.P. 35, 2558, 1964), it is necessary to minimize the angle of incidence as much as practicable and keep it approximately constant.
As described above, the conventional oblique incidence vacuum deposition process presents a very real problem with regard to the drop in the deposition efficiency, which leads to a rise in the production cost. This problem is aggravated particularly when a relatively expensive nonferrous metal such as Co, Co alloy or the like is used.
To solve the above mentioned problem, it has been proposed in Japanese unexamined Patent Publication No. 54 (1979)-12547 to locate a deposition material heated to a high temperature at a position transversely shifted from the center line of a cylindrical cooling can through which cooling water (usually at normal temperature) is passed. A substrate is moved along the curved surface of the cylindrical cooling can, and only the high-density portion of the vapor stream of the deposition metal is caused to impinge upon the curved surface of the flexible substrate. This method can give a deposition efficiency of about 20%.
With this method, to increase the effective deposition area upon which the high-density portion of the vapor stream impinges, it is necessary to increase the outer diameter of the cylindrical cooling can. However, increasing the outer diameter of the can naturally increase the size of the whole apparatus, resulting in higher equipment cost and greater maintenance requirements.
As disclosed in Japanese unexamined Patent Publication No. 53(1978)-95604, an attempt has also been made to remove the aforesaid cylindrical cooling can and use a plurality of guide rollers for guiding the substrate so that the substrate moves along a straight (not a curved) course between adjacent pairs of the guide rollers at an oblique angle with respect to the deposition material located below the guide rollers. The vapor stream of the deposition material is caused to impinge upon the straight surface of the substrate.
The method just described above can be conducted by use of a relatively small apparatus and can form a thin metal film at a high deposition efficiency. With this method, however, it is impossible to cool the substrate, which is heated by the deposition material heated for evaporation and the vapor stream generated. Accordingly, the deposition surface tends to develop wrinkles, greatly affecting the uniformity of the thickness of the deposited film and the smooth movement of the substrate.